Neuropsychiatric disorders may be etiologically complex and heterogeneous thereby making it difficult to identify risk factors for a particular neuropsychiatric disorder. Microarray techniques hold great promise for identifying risk factors for neuropsychiatric disorders such as schizophrenia (SZ) but have not yet generated widely reproducible results due to methodological differences between studies and the high risk of type I inferential errors.
Schizophrenia has a substantial genetic basis, but its biological underpinnings remain largely unknown. Early attempts to profile the expression of specific neurochemicals in blood and postmortem brain tissue detected several promising candidate risk factors for SZ that ultimately could not be substantiated. Subsequent progress in mapping the human genome increased the viability of candidate gene association studies. Most candidate genes have been targeted based on their expression within systems widely implicated in the disorder (e.g., dopamine and glutamate neurotransmitter systems), and this approach may be used for clarifying the nature of dysfunction within these recognized candidate pathways; however, it may not be optimal for identifying additional novel risk factors outside of these systems.
The advent of microarrays that can survey the entire expressed human genome has made it possible to simultaneously investigate the roles of several thousand genes in a disorder. Relative to traditional candidate gene studies predicated on existing disease models, microarray analysis is a less-constrained strategy that may foster the discovery of novel risk genes that otherwise would not come under study. Because gene expression can reflect both genetic and environmental influences, it may be particularly useful for identifying risk factors for a complex disorder such as SZ, which is thought to have a multifactorial polygenic etiology in which many genes and environmental factors interact. However, the simultaneous consideration of thousands of dependent variables also increases the likelihood of false-positive results. In short, microarrays hold great promise for identifying etiologic factors for SZ but run the risk of being too liberal and failing to provide replicable results.
Several groups have characterized gene expression profiles of SZ in postmortem tissue from the dorsolateral prefrontal cortex (DLPFC) of the brain, which has been consistently identified as dysfunctional in the illness. These studies have noted variable patterns of dysregulated gene expression in several domains, including G protein signaling, metabolism, mitochondrial function, myelination, and neuronal development. However, not all of these studies have reported significant alterations in each domain. Methodological differences, including ethnic and demographic disparities, alternative microarray platforms, and diverse methods of data analysis, as well as the high risk of false positives, have been cited as factors possibly contributing to this variability.
As a result, the gene expression profiles generated by existing methods are prone to the identification of false-positive results or risk factors that have little utility for diagnostic and predictive purposes.